arXiv subjects

Li-Sheng Geng

Publications and source records attributed to Li-Sheng Geng.

At least 163 records · Page 9Linked to original sources

arXiv2020

Revisiting the $Ω(2012)$ as a hadronic molecule and its strong decays

Recently, the Belle collaboration measured the ratios of the branching fractions of the newly observed $Ω(2012)$ excited state. They did not observe significant signals for the $Ω(2012) \to \bar{K} Ξ^*(1530) \to \bar{K} πΞ$ decay, and reported an upper limit for the ratio of the three body decay to the two body decay mode of $Ω(2012) \to \bar{K} Ξ$. In this work, we revisit the newly observed $Ω(2012)$ from the molecular perspective where this resonance appears to be a dynamically generated state with spin-parity $3/2^-$ from the coupled channels interactions of the $\bar{K} Ξ^*(1530)$ and $ηΩ$ in $s$-wave and $\bar{K} Ξ$ in $d$-wave. With the model parameters for the $d$-wave interaction, we show that the ratio of these decay fractions reported recently by the Belle collaboration can be easily accommodated.

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arXiv2020

Novel ansatz for charge radii in density functional theories

Charge radii are one of the most fundamental properties of atomic nuclei characterizing their charge distributions. Though the general trend as a function of the mass number is well described by the $A^{1/3}$ rule, some fine structures, such as the evolution along the calcium isotopic chain and the corresponding odd-even staggerings, are notoriously difficult to describe both in density functional theories and ab initio methods. In this letter, we propose a novel ansatz to describe the charge radii of calcium isotopes, by adding a correction term, proportional to the number of Cooper pairs, and determined by the BCS amplitudes and a single parameter, to the charge radii calculated in the relativistic mean field model with the pairing interaction treated with the BCS method. The new ansatz yields results consistent with data not only for calcium isotopes, but also for ten other isotopic chains, including oxygen, neon, magnesium, chromium, nickel, germanium, zirconium, cadmium, tin, and lead. It is remarkable that this ansatz with a single parameter can describe nuclear charge radii throughout the periodic table, particularly the odd-even staggerings and parabolic behavior. We hope that the present study can stimulate more discussions about its nature and relation with other effects proposed to explain the odd-even staggerings of charge radii.

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arXiv2020

Quadruply charmed baryons as heavy quark symmetry partners of the $D_{s0}^*(2317)$

Both unitary chiral theories and lattice QCD simulations show that the $DK$ interaction is attractive and can form a bound state, namely, $D^*_{s0}(2317)$. Assuming the validity of the heavy antiquark-diquark symmetry (HADS), the $Ξ_{cc}\bar{K}$ interaction is the same as the $DK$ interaction, which implies the existence of a $Ξ_{cc}\bar{K}$ bound state with a binding energy of $49-64$ MeV. In this work, we study whether a $Ξ_{cc}Ξ_{cc}\bar{K}$ three-body system binds. The $Ξ_{cc}Ξ_{cc}$ interaction is described by exchanging $π$, $σ$, $ρ$, and $ω$ mesons, with the corresponding couplings related to those of the $NN$ interaction via the quark model. We indeed find a $Ξ_{cc}Ξ_{cc}\bar{K}$ bound state, with quantum numbers $J^P=0^-$, $I=\frac{1}{2}$, $S=1$ and $C=4$, and a binding energy of $80-118$ MeV. It is interesting to note that this system is very similar to the well-known $NN\bar{K}$ system, which has been studied extensively both theoretically and experimentally. Within the same framework, we show the existence of a $NN\bar{K}$ state with a binding energy of $35-43$ MeV, consistent with the results of other theoretical works and experimental data, which serves as a consistency check on the predicted $Ξ_{cc}Ξ_{cc}\bar{K}$ bound state.

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arXiv2020

Theoretical interpretation of the $D^+_s \to π^+ π^0 η$ decay and the nature of $a_0(980)$

In a recent paper \cite{Ablikim:2019pit}, the BESIII collaboration reported the so-called first observation of pure $W$-annihilation decays $D^+_s \to a^+_0(980) π^0$ and $D_s^+ \to a^0_0(980)π^+$. The measured absolute branching fractions are, however, puzzlingly larger than those of other measured pure $W$-annihilation decays by at least one order of magnitude. In addition, the relative phase between the two decay modes is found to be about 180 degrees. In this letter, we show that all these can be easily understood if the $a_0(980)$ is a dynamically generated state from $\bar{K} K$ and $πη$ interactions in coupled channels. In such a scenario, the $D^+_s$ decay proceeds via internal $W$ emission instead of $W$-annihilation, which has a larger decay rate than $W$-annihilation. The proposed decay mechanism and the molecular nature of the $a_0(980)$ also provide a natural explanation to the measured negative interference between the two decay modes.

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arXiv2020

Towards the discovery of new physics with lepton-universality ratios of $b\to s\ell\ell$ decays

Tests of lepton-universality as rate ratios in $b\to s \ell\ell$ transitions can be predicted very accurately in the Standard Model. The deficits with respect to expectations reported by the LHCb experiment in muon-to-electron ratios of the $B\to K^{(*)}\ell\ell$ decay rates thus point to genuine manifestations of lepton non-universal new physics. In this paper, we analyse these measurements in the context of effective field theory. First, we discuss the interplay of the different operators in $R_K$ and $R_{K^*}$ and provide predictions for $R_{K^*}$ in the Standard Model and in new-physics scenarios that can explain $R_K$. We also provide approximate numerical formulas for these observables in bins of interest as functions of the relevant Wilson coefficients. Secondly, we perform frequentist fits to $R_{K}$ and $R_{K^*}$. The SM disagrees with these measurements at $3.7σ$ significance. We find excellent fits in scenarios with combinations of $\mathcal O_{9(10)}^\ell=\bar sγ^μb_L~\ellγ_μ(γ_5) \ell$ operators, with pulls relative to the Standard Model in the region of $4σ$. An important conclusion of our analysis is that a lepton-specific contribution to $\mathcal O_{10}$ is essential to understand the data. Under the hypothesis that new-physics couples selectively to the muons, we also present fits to other $b\to sμμ$ data with a conservative error assessment, and comment on more general scenarios. Finally, we discuss new lepton universality ratios that, if new physics is the origin of the observed discrepancy, should contribute to the statistically significant discovery of new physics in the near future.

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arXiv2020

Renormalizability of leading order covariant chiral nucleon-nucleon interaction

In this work, we study the renormalization group invariance (RGI) of the recently proposed covariant power counting (PC) scheme in the case of nucleon-nucleon scattering [Chin.Phys. C42 (2018) 014103] at leading order (LO). We show that unlike the LO Weinberg case, RGI is satisfied in the $^3P_{0}$ channel, because a term of $pp'$ appears naturally in the covariant PC scheme at LO. Another interesting feature is that the $^1S_{0}$ and $^3P_{1}$ channels are correlated. Fixing the two relevant low energy constants by fitting to the $^1S_{0}$ phase shifts at $T_\mathrm{lab.}=10$ and $25$ MeV with a cutoff $Λ$ of $400-650$ MeV, the $^3P_{1}$ phase shifts can be described relatively well. In the limit of $Λ\rightarrow \infty$, the $^1S_0$ channel becomes cutoff independent, while RGI is lost in the $^3P_{1}$ channel, consistent with the Wigner bound and the previous observation that the $^{3}P_1$ channel better be treated perturbatively. As for the $^1P_{1}$ and $^3S_{1}$-$^3D_{1}$ channels, RGI is satisfied, similar to the Weinberg approach.

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arXiv2019

Revisiting the new-physics interpretation of the $b\to cτν$ data

We revisit the status of the new-physics interpretations of the anomalies in semileptonic $B$ decays in light of the new data reported by Belle on the lepton-universality ratios $R_{D^{(*)}}$ using the semileptonic tag and on the longitudinal polarization of the $D^*$ in $B\to D^*τν$, $F_L^{D^*}$. The preferred solutions involve new left-handed currents or tensor contributions. Interpretations with pure right-handed currents are disfavored by the LHC data, while pure scalar models are disfavored by the upper limits derived either from the LHC or from the $B_c$ lifetime. The observable $F_L^{D^*}$ also gives an important constraint leading to the exclusion of large regions of parameter space. Finally, we investigate the sensitivity of different observables to the various scenarios and conclude that a measurement of the tau polarization in the decay mode $B\to Dτν$ would effectively discriminate among them.

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arXiv2019

Strong decays of the explicitly exotic doubly charmed $DDK$ bound state

Nowadays, it is generally accepted that the $DK$ interaction in isospin zero is strongly attractive and the $D_{s0}^*(2317)$ can be described as a $DK$ molecular state. Recent studies show that the three-body $DDK$ system binds as well with a binding energy about 60$\sim$70 MeV. The $DDK$ bound state has isospin $1/2$ and spin-parity $0^-$. If discovered either experimentally or in lattice QCD, it will not only provide further support on the molecular nature of the $D_{s0}^*(2317)$, but also provide a way to understand other exotic hadrons expected to be of molecular nature. In the present work, we study its two-body strong decay widths via triangle diagrams. We find that the partial decay width into $DD_sπ$ is at the order of $2\sim3$ MeV, which seems to be within the reach of the current experiments such as BelleII. As a result, we strongly recommend this decay channel of the $DDK$ bound state to be searched for experimentally.

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arXiv2019

Exploring the molecular scenario of $P_c(4312)$, $P_c(4440)$, and $P_c(4457)$

In the present work, we assign the newly observed $P_c(4312)$ as a $I(J^P)= \frac{1}{2} (\frac{1}{2})^-$ molecular state composed of $Σ_c \bar{D}$, while $P_c(4440)$ and $P_c(4457)$ as $Σ_c \bar{D}^\ast$ molecular states with $I(J^P)= \frac{1}{2} (\frac{1}{2})^-$ and $\frac{1}{2} (\frac{3}{2})^-$, respectively. In this molecular scenario, we investigate the $P_c\to J/ψp$ process of these three states and further predict the ratios of the $\mathcal B(P_c\to J/ψp)$ and those of $\mathcal B(Λ_b\to P_c K)$ between these three states, which could serve as a crucial test of the present molecular scenario.

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arXiv2019

Model independent determination of the spins of the $P_{c}$(4440) and $P_{c}$(4457) from the spectroscopy of the triply charmed dibaryons

The LHCb collaboration has recently observed three narrow pentaquark states --- the $P_c(4312)$, $P_c(4440)$, and $P_c(4457)$ ---that are located close to the $\bar{D} Σ_c$ and $\bar{D}^* Σ_c$ thresholds. Among the so-far proposed theoretical interpretations for these pentaquarks, the molecular hypothesis seems to be the preferred one. Nevertheless, in the molecular picture the spins of the $P_c(4440)$ and $P_c(4457)$ have not been unambiguously determined yet. In this letter we point out that heavy antiquark-diquark symmetry induces a model-independent relation between the spin-splitting in the masses of the $P_c(4440)$ and $P_c(4457)$ $\bar{D}^* Σ_c$ pentaquarks and the corresponding splitting for the $0^+$ and $1^+$ $Ξ_{cc} Σ_c$ triply charmed dibaryons. This is particularly relevant owing to a recent lattice QCD prediction of the $1^+$ triply charmed dibaryon, which suggests that a calculation of the mass of its $0^+$ partner might be within reach. This in turn would reveal the spins of the $P_c(4440)$ and $P_c(4457)$ pentaquarks, providing a highly nontrivial test of heavy-quark symmetry and the molecular nature of the pentaquarks. Furthermore, the molecular interpretation of the hidden-charm pentaquarks implies the existence of a total of ten triply charmed dibaryons as $Ξ_{cc}^{(*)} Σ_c^{(*)}$ molecules, which, if confirmed in the lattice, will largely expand our understanding of the non-perturbative strong interaction in the heavy-quark sector.

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arXiv2019

$DK$, $DDK$, and $DDDK$ molecules--understanding the nature of the $D_{s0}^*(2317)$

The $DK$ interaction is strong enough to form a bound state, the $D_{s0}^*(2317)$. This in turn begs the question of whether there are bound states composed of several charmed mesons and a kaon. Previous calculations indicate that the three-body $DDK$ system is probably bound, where the quantum numbers are $J^P = 0^{-}$, $I=\tfrac{1}{2}$, $S = 1$ and $C = 2$. The minimum quark content of this state is $cc\bar{q}\bar{s}$ with $q=u,d$, which means that, if discovered, it will be an explicitly exotic tetraquark. In the present work. we apply the Gaussian Expansion Method to study the $DDDK$ system and show that it binds as well. The existence of these three and four body states is rather robust with respect to the $DD$ interaction and subleading (chiral) corrections to the $DK$ interaction. If these states exist, it is quite likely that their heavy quark symmetry counterparts exist as well. These three-body $DDK$ and four-body $DDDK$ molecular states could be viewed as counterparts of atomic nuclei, which are clusters of nucleons bound by the residual strong force, or chemical molecules, which are clusters of atoms bound by the residual electromagnetic interaction.

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arXiv2019

Emergence of a complete heavy-quark spin symmetry multiplet: seven molecular pentaquarks in light of the latest LHCb analysis

A recent analysis by the LHCb collaboration suggests the existence of three narrow pentaquark-like states --- the $P_c(4312)$,$P_c(4440)$ and $P_c(4457)$ --- instead of just one in the previous analysis (the $P_c(4450)$). The closeness of the $P_c(4312)$ to the $\bar{D} Σ_c$ threshold and the $P_c(4440)$/$P_c(4457)$ to the $\bar{D}^* Σ_c$ one suggests a molecular interpretation of these resonances. We show that these three pentaquark-like resonances can be naturally accommodated in a contact-range effective field theory description that incorporates heavy-quark spin symmetry. This description leads to the prediction of all the seven possible S-wave heavy antimeson-baryon molecules (that is, there should be four additional molecular pentaquarks in addition to the $P_c(4312)$, $P_c(4440)$ and $P_c(4457)$), providing the first example of a heavy-quark spin symmetry molecular multiplet that is complete. If this is confirmed, it will not only give us an impressive example of the application of heavy-quark symmetries and effective field theories in hadron physics: it will also uncover a clear and powerful ordering principle for the molecular spectrum, reminiscent of the SU(3)-flavor multiplets to which the light hadron spectrum conforms.

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arXiv2019

Bound state formation in the $DDK$ system

We study the $DDK$ system in a coupled channel approach, by including $DD_sη$ and $DD_sπ$, and find that the dynamics involved in the system forms a bound state with isospin $1/2$ and mass $4140$ MeV when one of the $DK$ pair is resonating in isospin 0, forming the $D^*_{s0}(2317)$. The state can be interpreted as a $DD^*_{s0}(2317)$ molecule like state with exotic quantum numbers: doubly charged, doubly charmed, and with single strangeness.

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arXiv2019

Heavy-quark spin and flavour symmetry partners of the X(3872) revisited: what can we learn from the one boson exchange model?

Heavy-quark symmetry as applied to heavy hadron systems implies that their interactions are independent of their heavy-quark spin (heavy-quark spin symmetry) and heavy flavour contents (heavy flavour symmetry). In the molecular hypothesis the $X(3872)$ resonance is a $1^{++}$ $D^*\bar{D}$ bound state. If this is the case, the application of heavy-quark symmetry to a molecular $X(3872)$ suggests the existence of a series of partner states, the most obvious of which is a possible $2^{++}$ $D^*\bar{D}^*$ bound state for which the two-body potential is identical to that of the $1^{++}$ $D^*\bar{D}$ system, the reason being that these two heavy hadron-antihadron states have identical light-spin content. As already discussed in the literature this leads to the prediction of a partner state at $4012\,{\rm MeV}$, at least in the absence of other dynamical effects which might affect the location of this molecule. However the prediction of further heavy-quark symmetry partners cannot be done solely on the basis of symmetry and requires additional information. We propose to use the one boson exchange model to fill this gap, in which case we will be able to predict or discard the existence of other partner states. Besides the isoscalar $2^{++}$ $D^*\bar{D}^*$ bound state, we correctly reproduce the location and quantum numbers of the isovector hidden-bottom $Z_b(10610)$ and $Z_b(10650)$ molecular candidates. We also predict the hidden-bottom $1^{++}$ $B^*\bar{B}^*$ and $2^{++}$ $B^*\bar{B}^*$ partners of the $X(3872)$, in agreement with previous theoretical speculations, plus a series of other states. The isoscalar, doubly charmed $1^+$ $D D^*$ and $D^* D^*$ molecules and their doubly bottomed counterparts are likely to bind, providing a few instances of explicitly exotic systems.

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arXiv2019

Heavy Baryon-Antibaryon Molecules in Effective Field Theory

We discuss the effective field theory description of bound states composed of a heavy baryon and antibaryon. This framework is a variation of the ones already developed for heavy meson-antimeson states to describe the $X(3872)$ or the $Z_c$ and $Z_b$ resonances. We consider the case of heavy baryons for which the light quark pair is in S-wave and we explore how heavy quark spin symmetry constrains the heavy baryon-antibaryon potential. The one pion exchange potential mediates the low energy dynamics of this system. We determine the relative importance of pion exchanges, in particular the tensor force. We find that in general pion exchanges are probably non-perturbative for the $Σ_Q \barΣ_Q$, $Σ_Q^* \barΣ_Q$ and $Σ_Q^* \barΣ_Q^*$ systems, while for the $Ξ_Q' \barΞ_Q'$, $Ξ_Q^* \barΞ_Q'$ and $Ξ_Q^* \barΞ_Q^*$ cases they are perturbative If we assume that the contact-range couplings of the effective field theory are saturated by the exchange of vector mesons, we can estimate for which quantum numbers it is more probable to find a heavy baryonium state. The most probable candidates to form bound states are the isoscalar $Λ_Q \barΛ_Q$, $Σ_Q \barΣ_Q$, $Σ_Q^* \barΣ_Q$ and $Σ_Q^* \barΣ_Q^*$ and the isovector $Λ_Q \barΣ_Q$ and $Λ_Q \barΣ_Q^*$ systems, both in the hidden-charm and hidden-bottom sectors. Their doubly-charmed and -bottom counterparts ($Λ_Q Λ_Q$, $Λ_Q Σ_Q^{(*)}$, $Σ_Q^{(*)} Σ_Q^{(*)}$) are also good candidates for binding.

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arXiv2018

Magnetic moments of the spin-1/2 singly charmed baryons in covariant baryon chiral perturbation theory

Recent experimental advances have reignited theoretical interests in heavy-flavor hadrons. In this work, we study the magnetic moments of the spin-1/2 singly charmed baryons up to the next-to-leading order in covariant baryon chiral perturbation theory with the extended-on-mass-shell renormalization scheme. The pertinent low energy constants $g_{1-4}$ are fixed with the help of the quark model and the heavy quark spin flavor symmetry, while the remaining $d_2$, $d_3$, $d_5$ and $d_6$ are determined by fitting to the lattice QCD pion-mass dependent data. We study the magnetic moments as a function of $m_π^2$ and compare our results with those obtained in the heavy baryon chiral perturbation theory. We find that the loop corrections induced by the anti-triplet states are dominated by the baryon pole diagram. In addition, we predict the magnetic moments of the spin-1/2 singly charmed baryons and compare them with those of other approaches.

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arXiv2018

Meson-baryon scattering up to the next-to-next-to-leading order in covariant baryon chiral perturbation theory

We study the scattering of a pseudoscalar meson off one ground state octet baryon in covariant baryon chiral perturbation theory (BChPT) up to the next-to-next-to-leading order. The inherent power counting breaking terms are removed within extended-on-mass-shell scheme. We perform the first combined study of the pion-nucleon and kaon-nucleon scattering data in covariant BChPT and show that it can provide a reasonable description of the experimental data. In addition, we find that it is possible to fit the experimental baryon masses and the pion-nucleon and kaon-nucleon scattering data simultaneously at this order, thus providing a consistent check on covariant BChPT. We compare the scattering lengths of all the pertinent channels with available experimental data and those of other approaches. In addition, we have studied the leading order contributions of the virtual decuplet and found that they can improve the description of the $πN$ phase shifts near the $Δ(1232)$ peak, while they have negligible effects on the description of the $K N$ phase shifts.

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arXiv2018

Covariant chiral nucleon-nucleon contact Lagrangian up to order $\mathcal{O}(q^4)$

We adopt a covariant version of the naive dimensional analysis and construct the chiral two-nucleon contact Lagrangian constrained by Lorentz, parity, charge conjugation, hermitian conjugation, and chiral symmetries. We show that at $\mathcal{O}(q^0)$, $\mathcal{O}(q^2)$, $\mathcal{O}(q^4)$, where $q$ denotes a generic small quantity, there are 4, 13, and 23 terms, respectively. We find that by performing $1/m_N$ expansions, the covariant Lagrangian reduces to the conventional non-relativistic one, which includes 2, 7, and 15 terms at each corresponding order.

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